This invention relates, in general, to methods of processing a semiconductor wafer, and more particularly to methods of thinning a semiconductor wafer.
Semiconductor wafer thinning techniques have been developed in response to the ever increasing demand for smaller, higher performance semiconductor devices. For example, semiconductor devices operated at high speeds generate large amounts of heat. This heat must be removed from the semiconductor device to prevent device failure due to heat stress and to prevent degradation of the frequency response due to a decrease in carrier mobility. One way to enhance thermal transfer away from the semiconductor device, thereby mitigating any deleterious temperature effects, is by thinning the semiconductor wafer in which the device is fabricated. Other reasons for thinning a semiconductor wafer include dimensional packaging constraints, optimization of transmission line characteristics, and formation of via holes.
Typically, semiconductor devices are thinned while the devices are in wafer form. A consequence of thinning a semiconductor wafer is to increase its fragility. Thus, a semiconductor wafer is generally mounted to a submount prior to thinning, wherein the submount provides structural support as well as protection for the wafer surface. A first step in mounting a semiconductor wafer to the submount is coating a major surface of the wafer with an adhesive material. The adhesive coated major surface is bonded to the submount, thereby forming a bonded wafer. In addition to providing structural support for the semiconductor wafer the submount may serve as a handle. An example of a technique for applying an adhesive material to a semiconductor wafer and mounting the semiconductor wafer to a submount may be found in U.S. Pat. No. 3,475,867 entitled "Processing of Semiconductor Wafers," and in U.S. Pat. No. 3,492,763 entitled "Method and Apparatus for Mounting Semiconductor Slices," respectively.
The bonded wafer is thinned by either mechanically grinding or chemically etching an exposed surface of the semiconductor wafer. The adhesive material bonding the semiconductor wafer to the submount must provide a bond capable of withstanding the shearing forces generated by mechanical grinding, the reactivity of a chemical etchant, and temperatures associated with semiconductor wafer processing.
Although methods for thinning semiconductor wafers are known, they are not without limitations. For example, the step of mounting a semiconductor wafer to a submount requires expensive coating and bonding equipment and increases the overall processing time for manufacturing a semiconductor device, i.e. increases the cycle time. Further, the use of liquid adhesive materials increases the potential for introducing contaminants into the process area. In addition, the processing temperatures which the bonded wafer encounters must remain below the melting temperature of the adhesive material. Other limitations include the warping or bowing of semiconductor wafers due to mismatches between the coefficients of thermal expansion of the wafer and the submount, complications in automatic wafer handling due to the thickness of the bonded wafer, and having an unsupported wafer upon separation of the wafer and the submount.
Accordingly, it would be advantageous to have a method for thinning a semiconductor wafer having an optimized number of processing steps and which is compatible with automated wafer handling systems. The method should decrease the cycle time required for processing a semiconductor wafer and be capable of employing either mechanical grinding, chemical etching, or a combination of the two. Further, it is desirable that the method provide a handle to eliminate any unsupported wafer handling that may occur after the wafer has been thinned and that the handle continue to provide support at temperatures used for sputter deposition of metals.